1. Field of the Invention
This invention relates to a solid electrolytic capacitor which has good capacitor characteristics, particularly good high frequency characteristics, and high reliability under high temperature and high humidity conditions. The invention also relates to a method for manufacturing such a capacitor.
2. Description of the Prior Art
In recent years, digitalization in the electric and electronic fields has been conspicuous. This entails a strong demand for capacitors which exhibit a low impedance in a high frequency range and are small in size and large in capacitance.
Known capacitors which are used in high frequency ranges include, for example, plastic film capacitors, mica capacitors, layer-built ceramic capacitors and the like. These capacitors are disadvantageously so large in size that a large capacitance is difficult to obtain.
On the other hand, a certain type of electrolytic capacitor is known as having a large capacitance. This type of capacitor includes, for example, an aluminium dry electrolytic capacitor and an aluminium or tantalum solid electrolytic capacitor. These electrolytic capacitors are advantageous in that since an anodized film serving as a dielectric layer can be formed very thinly, a large capacitance can be realized. However, the anodized film is liable to undergo damages, so that it becomes necessary to provide an electrolyte between the anodized film and a cathode in order to repair the damages.
With aluminium dry electrolytic capacitors, anode and cathode aluminium foils which have been, respectively, etched, are convolutely wound through a separator and a liquid electrolyte is impregnated in the separator. This presents the problems that since the liquid electrolyte is ion conductive in nature with a large specific resistance, so that the loss (tan .delta.) becomes great with very poor frequency and temperature characteristics. In addition, the leakage and evaporation of the liquid electrolyte inevitably occurs, thus leading to a decrease of the capacitance and an increase of the loss with time.
With the tantalum solid electrolytic capacitor, manganese dioxide is used as the electrolyte. Accordingly, the problems on the temperature characteristic and the changes of the capacitance and loss in relation to the time can be overcome. However, the relatively high specific resistance of manganese dioxide results in the loss and a frequency characteristic of impedance poorer than those of the layer-built ceramic capacitors and film capacitors.
In order to solve the above problems, there have been recently proposed solid electrolytic capacitors wherein highly conductive polymers, which contain an anion of a support electrolyte as a dopant and which are obtained by electrolytically polymerizing heterocyclic monomers such as pyrrole, thiophene and the like, are formed on an anode through a conductive underlying layer (Japanese Kokai Patent Application Nos. 62-181415 and 63-173313). The solid electrolytic capacitor has good frequency and temperature characteristics.
However, the known solid electrolytic capacitors are liable to suffer degradation of the anodized oxide layer, e.g. an alumina layer, under high temperature and high humidity conditions. This will cause a lowering of the capacitance and an increase of the loss, making it difficult to obtain a stable capacitor which has high reliability.